For me, the strongest arguments in favor of an HJ are the gospel scenes in which Jesus fights with his family and can't mention his mother without attacking her: Mark 3:31, Mark 6:3, Mark 15:40, Matthew 10:35, Luke 11:27. John 2:3, contrast only John 19:26. Suddenly you could suspect a real human in there somewhere.
It's more than that.
In both Galatians 1:18-9 and 1 Corinthians 9:5 the “
brothers of the Lord” are mentioned alongside and separate from other believers. The context clearly implies that they are blood brothers and St. Paul knew one of them, James, whom he mentions in passing. His admission of acquaintance with James in this passage doesn't even help bolster his claim to independent apostleship through "revelation", such that he seems to grudgingly admit that he did receive information about Jesus from his brother.
What's intriguing, is that Paul's audience is already familiar with James and has been under the influence of a circumcision faction dispatched by James that is eroding trust in Paul's authority.
The great difficulty for Paul was in trying to convince people that his interpretation of Jesus was equal to that of the other apostles who had actually known him in life, especially when their number included Jesus's actual family members who'd been raised with him. This was embarrassing for Paul.
The existence of Jesus’ brother is further strengthened by the fact he is attested outside of Christian texts – in Josephus,
Antiquities of the Jews, XX.200-203:
“When, therefore, Ananus was of this disposition, he thought he had now a proper opportunity. Festus was now dead, and Albinus was upon the road; so he assembled the Sanhedrin of judges, and brought before them the brother of Jesus, who was called Messiah (τον αδελφον Ιησου του λεγομενου Χριστου), whose name was James, and some others. And, when he had formed an accusation against them as breakers of the law, he delivered them to be stoned.
But as for those who seemed the most equitable of the citizens, and such as were the most uneasy at the breach of the laws, they disliked what was done; they also sent to the king, desiring him to send to Ananus that he should act so no more, for that what he had already done was not to be justified; nay, some of them went also to meet Albinus, as he was upon his journey from Alexandria, and informed him that it was not lawful for Ananus to assemble a Sanhedrin without his consent. Whereupon Albinus complied with what they said, and wrote in anger to Ananus, and threatened that he would bring him to punishment for what he had done; on which king Agrippa took the high priesthood from him when he had ruled but three months, and made Jesus, the son of Damneus, high priest.”
Unlike the other reference to Jesus in Josephus – the so-called “
Testimonium Flavianum” at
Ant. XVIII.63-4 – this reference is near universally regarded as genuine and as referring to Jesus Christ and his brother James.
Josephus was a younger contemporary of James, being around 25 when James was executed, and a fellow citizen of Jerusalem, a small city of around 80,000 inhabitants (see Josephus,
Life, III). As such, Josephus is likely speaking from first-hand knowledge and not merely as a historian.
So, in sum: Paul physically met Jesus’ brother and spoke with him on different occasions. Josephus was a younger contemporary of the same brother and witnessed the events of James’ execution in Jerusalem where he lived. James the brother of Jesus existed, which means that his brother Jesus of Nazareth also must have existed.
No mythicist arguments have ever or can ever get around this.